The invention relates to a system for sensing a physical characteristic of a rotating wheel of a vehicle, and in particular, to a method and apparatus for encoding data indicating the pressure and temperature of the gas within the vehicle wheel.
It is commonly known to provide vehicles with wheels that include a tire, typically made of rubber or some other elastomeric material. The tire is usually hollow and is usually filled to a given pressure with a gas (commonly air). It is very important that the tire of the vehicle be inflated to the correct pressure for the particular tire. Improper inflation of a vehicle tire adversely effects the wear of the tire and diminishes the efficiency of the vehicle. Therefore, it is important that the vehicle operator check the tires on a regular basis to ensure proper inflation of the tires. However, the process of checking and adding to or subtracting from the air in the tire can be tedious and inconvenient for the typical motor vehicle operator. Moreover, the air pressure in a given tire can vary significantly depending upon the changes in the ambient temperature
Various systems have been developed to make monitoring of tire pressure more convenient and easy. For example, U.S. Pat. No. 4,918,423 describes a tire inspection device for measuring the pressure and temperature of the air in the tires The device includes a pressure sensor switch 9 connected to a coil 8, and a hand-held detecting rod 28. If the tire internal pressure has the proper value, the sensor 9 turns on and a green lamp on the detecting rod 28 lights. If the tire internal pressure has the improper values the sensor 9 turns off and a red lamp on the detecting rod 28 lights.
Another example of a system for detecting tire pressure is shown and described in U.S. Pat. No. 4,768,375 (Eckardt et al.). The system illustrated in Eckardt et al. is passive in that it does not require a person to take any action to detect the tire pressure. The Eckardt et al. system can also detect tire pressure while the vehicle wheel is rotating.